Hmm... there are two methods of going fullscreen in Windows for Demon, the success of which seems to vary from system to system: F11 and Alt+Enter. For me, F11 doesn't really work (it goes full screen, but doesn't get bigger), but Alt+Enter does. If neither work for you, can you tell me what happens when you try them, if anything at all?
well, last time I played I tried both and they didnt seem to do anything, I even tried closing and opening
but idk why I opened it just now to try it again to tell u if I noticed anything and it is full screen, well, it's kinda the same size but black borders cover the parts of the screen that it didnt fit to, I pressed F11 and that returned it to small and back to the black outline once I pressed it again
so after that I tried pressing alt+enter and it actually made it full screen, this is so weird, it didnt work any of the times before, maybe it was something of my computer? sometimes a ctrl or alt or shift key remains pressed on my pc, so maybe that's what happened